Light sensitive recording materials may suffer from a phenomenon known as halation which causes degradation in the quality of the recorded image. Such degradation may occur when a fraction of the imaging light which strikes the photosensitive layer is not absorbed but passes through to the film base on which the photosensitive layer is coated. A portion of the light reaching the base may be reflected back to strike the photosensitive layer from the underside. Light thus reflected may, in some cases, contribute significantly to the total exposure of the photosensitive layer. Any particulate matter in the photosensitive element, including silver salts, may cause light passing through the element to be scattered. Scattered light which is reflected from the film base will, on its second passage through the photosensitive layers cause exposure over an area adjacent to the point of intended exposure. It is this adjacent exposure effect which leads to image degradation. Silver halide based photographic materials (including photothermographic materials) are particularly prone to this form of image degradation since the photosensitive layers contain light scattering particles. The effect of light scatter on image quality is well documented and is described, for example, in T. H. James "The Theory of the Photographic Process", 4th Edition, Chapter 20, Macmillan 1977.
It is common practice to minimize the effects of light scatter by including a light absorbing layer within the photographic element. To be effective, the absorption of this layer must be at the same wavelengths as the sensitivity of the photosensitive layer. In the case of imaging materials coated on transparent base, a light absorbing layer is frequently coated on the reverse side of the base from the photosensitive layer. Such a coating, known as an "antihalation layer", effectively prevents reflection of any light which has passed through the photosensitive layer.
A similar effect may be achieved by a light absorbing layer interposed between the photosensitive layer and the base. This construction, described as an "antihalation underlayer" is applicable to photosensitive coatings on transparent or non-transparent bases. It is also possible to improve image quality by coating a light absorbing layer above the photosensitive layer of a photographic or photothermographic element. Coatings of this kind, described in U.S. Pat. Nos. 4,581,323 and 4,312,941, prevent multiple reflections of scattered light between the internal surfaces of a photosensitive element.
Photothermographic antihalation systems for infrared materials have been described previously. However, these usually had some disadvantages. A strippable antihalation coating of infrared absorbing pigment such as carbon black is described in U.S. Pat. Nos. 4,477,562 and 4,409,316. A strippable layer would generally have adhesion difficulties in processes such as coating, converting and packaging and also generates a sheet of pigmented waste material. For these reasons, it is not a desirable solution to the problem.
European Patent Application 0 377 961 and U.S. Pat. No. 4,581,325 describe infrared antihalation systems for photographic and photothermographic materials incorporating polymethine and holopolar dyes respectively. However, these dyes although having good infrared absorbance, have visible absorbance that is too high for use in subsequent exposures or viewing an image.
An antihalation system that has a high infrared absorbance before processing and a low visible absorbance after processing is the thermal-dye-bleach construction described in European Patent Application 0 403 157. The bleaching, infrared antihalation system uses a polymethine dye which is converted to a colorless derivative on heat processing. However, the system is not heat stable and as the dye decomposes,the infrared absorbance decreases with time.
Indolenine dyes have been described as infrared antihalation dyes in silver halide, photographic materials in U.S. Pat. Nos. 2,895,955; 4,882,265; 4,876,181; 4,839,265 and 4,871,656 and Japanese Patent Application J63 195656. Infrared absorbing indolenine dyes have been described for electrophotography in U.S. Pat. No. 4,362,800 and for optical laser recording material in Japanese Patent Applications J6 2082-082A and J6 3033-477.
Water insoluble indolenine dyes have been used in photothermographic systems as weak sensitizing dyes in U.S. patent application No. U.S. Ser. No. 07/846,919 filed Apr. 13, 1992 and as a combined supersensitizer-antihalation system in Japanese Patent Application J4 182640.
However, the combination of infrared absorbing, water soluble indolenine dyes in a hydrophilic binder for photothermographic antihalation systems has not been described.